The reintroduction of the Aldabra rail Dryolimnas cuvieri aldabranus to Picard Island, Aldabra Atoll

Abstract

The rallid genus Dryolimnas is endemic to western Indian Ocean islands. Formerly widespread, it is now restricted to Aldabra Atoll (the Aldabra Rail D. cuvieri aldabranus) and Madagascar (the White-throated Rail D. c. cuvieri). Before this project began, the Aldabra Rail was restricted to Polymnie, Malabar and lIe aux Cedres, having been extirpated from Grande Terre and Picard by Domestic Cats Felis catus (still found on Grande Terre) and humans. In 1999, following IUCN guidelines, I captured 20 Aldabra Rails and brought them to now Cat-free Picard. Two Rails died in captivity, an indirect result of incorrect sexing based on a published field sexing method; a subsequent genetic sexing technique confirmed the unreliability of sexing Aldabra Rails in the field. All 18 remaining birds were released on Picard and survived beyond the first breeding season. Eight pairs had bonded and successfully reared a minimum of 13 chicks within five months of release. Eleven monitored pairs reared 20 chicks in 2000/2001, with one-year-old birds breeding successfully. Average chick production was significantly higher on Picard than on Malabar in both breeding seasons. The Picard population at the end of the 2000/2001 breeding season was at least 51, an increase of 283% in 18 months. There are excellent prospects for continued, exponential population growth until the population reaches carrying capacity (ca 1000 pairs), predicted to be between 2007 and 2009. Limited genetic data suggest some population structure between extant populations, and the Picard population is genetically healthy relative to that of Malabar. There has been no substantial change 111 the Aldabra Rail population since the 1960s. However, I have revised an earlier population estimate downwards to ca 3500 pairs. I also estimate the floating population on Malabar was around 1700 Rails in early 2000. A population model of the Malabar Rails predicts a likely minimum annual adult survival rate of 85 %, with a fluctuating floater population. It also shows the population to be very robust to the removal of territorial birds for other proposed reintroductions, but highly vulnerable to the establishment of a predator. The soft release protocol (including a period of acclimatisation before release and supplementary feeding, as required, after release) is believed to have played a crucial role in the success of the reintroduction by allowing individuals to acclimatise and providing an energetic cushion between release and self-sufficiency. A soft release is recommended as the conservative and precautionary method of choice for avian reintroductions and translocations.